The present invention relates to a method of sulfide tarnish inhibiting of silver alloys, as well as to silver alloys which are treated in accordance with the above method.
It is well known that chemical stability and in particular corrossion stability of silver is the main factor why it is widely used for manufacture of equipment parts in chemical industry, and also an important factor of its use in many other fields. For example, only its chemical stability explains the wide usage of silver for making mirrors, coins, silverware, bearing, electrical contacts, as well as its usage in jewelry and dentistry. Corrosion stability of silver can be explained mainly by its position in the row of potentials. To smaller extent, it is also a result of its ability to form a protective film of oxide (Ag.sub.2 O) which, however, is not dense and stable enough, and has a porosity.
At the same time, it is well known that silver is not stable in presence of hydrogen sulfide (H.sub.2 S) and other sulfide components even in small concentrations. The surface of silver very quickly covers with a film of silver sulfide (Ag.sub.2 S). The initially light color of film becomes darker and soon turns dark, then greenishbrown, and finally black. The rate of tarnishing of silver-copper alloys increases with greater copper content, because in addition to silver sulfide, black copper sulfide (Cu.sub.2 S) is also formed. The rate of tarnishing of silver and silver alloys accelerates with increasing in humidity of air and concentration of sulfur compounds.
Sulfide film can be removed from the surface by a chemical treatment, by polishing, or their combination. To prevent tarnishing, it is possible to cover the surfaces of silver products by film of other stable metals, such as Rhodium, by film of oxides of Aluminum, Beryllium, Technetium and Zirconium, or by cathodic passivation in salts of Chromium and other metals. These methods can increase the tarnish resistance of silver up to five times. However, it was found that main disadvantage of those methods is that any accidental scratch will expose the fresh silver surface and therefore initiate the tarnishing process.
There have been a lot of attempts to produce a "stainless" silver alloy. They, however, failed since they involved adding of another metal in big quantities. The problem is that such addition of big quantities of metal simultaneously changes the karat composition, appearance and properties of silver.